a) Field of the Invention
The present invention relates to a cleaning method and apparatus, more particularly to a cleaning method and apparatus suitable for cleaning the surface of a plane such as the surface of a semiconductor wafer.
At LSI manufacturing steps, fine particles or stains on the surface of a semiconductor wafer, an LCD, a solar battery or the like lower the final product yield considerably. Wafer surface cleaning is therefore important, and environmental contamination to be caused by cleaning is also an important issue.
b) Description of the Related Arts
Various cleaning methods have been proposed heretofore. Surface cleaning methods used heretofore for semiconductor wafers are summarized below.
Chemical Cleaning or Solvent Cleaning
This method removes surface stains by a chemical reaction or dissolution. Water, acid, organic solvent, freon, or the like is used. It is necessary to select proper solvents effective for stains, soils, and etc. to be removed. Ultrasonic cleaning may be used together to improve the physical cleaning power. It is necessary to use high purity solvent in order not to leave stains or soils on the cleaned object surface.
Water of high purity can be easily obtained and a great amount of water can be used cost effectively, but water left on the surface may cause future contamination, and the type of soils, stains, and the like to be dissolved by water is limited.
Many of other effective solvents contaminate the environment if they are left in the environment. In circulating solvents in order to prevent environmental contamination, it is difficult to refine circulating solvents cost effectively. If cleaning is repeated using the same solvent, contaminated substances are accumulated in this solvent and they may attach to the cleaned surface, resulting in defective products.
Fine Ice Particle Blowing
There is known a method of blowing fine ice particles to remove fine foreign particles or stains on an object surface. However, the diameter of a fine ice particle manufacturable presently is not sufficiently so small as to easily remove fine foreign particles of 1 .mu.m or smaller.
Fine CO.sub.2 Particle Blowing
There is also known a method of blowing fine dry ice particles to remove fine foreign particles or stains on an object surface. However, it is very difficult to extract hydro carbon compounds from carbon dioxide gas to a very low concentration. If CO.sub.2 is cooled and blown, hydro carbon compounds are condensed and adhered to the cleaned surface. CO.sub.2 may become a C contamination source.
Gas Blowing
There is a method of blowing gas to an object surface of a solid to clean it. However, a gas interface layer having a very slow flow is formed on the solid surface. Such a slow gas flow has a weak power of removing fine particles, being difficult to remove fine particles of 1 .mu.m or smaller. The bond strength of a particle to the surface is proportional to its diameter, and the particle removing power is proportional to the square of a particle diameter.
Argon Gas Blowing at Extremely Low Temperature
A method of blowing argon gas or mixed gas containing argon gas cooled to an extremely low temperature to an object surface, is also known. By releasing gas from a nozzle to a vacuum chamber, the gas is adiabatically expanded rapidly and lowers its temperature. At this lowered temperature, solid argon is formed and solid argon fine particles collide against an object surface.
There has been proposed a method of transforming, for example, an argon gas to a solid argon by cooling the gas containing argon gas under a pressurized state to a temperate higher than the liquefying temperature of argon gas specific to its pressure, and by jetting the gas out of the nozzle to a vacuum chamber.
Impurities having a liquefying temperature higher than argon can be removed prior to cooling argon gas to the liquefying temperature. With this method, however, the number of solid argon fine particles is small, providing only a weak cleaning power.
Argon is an inert element and a bad effect rarely occurs even if it attaches to the solid surface. The solidifying temperature of argon is relatively high, and it is relatively easy to obtain solid argon by cooling argon gas.
However, a practical cleaning technique using solid argon fine particles has not been developed as yet which provides a high cleaning power.